It is one of the functions of a pulse code modulation (PCM) channel bank to receive voice frequency (VF) signals from a plurality of VF lines and deliver them encoded and multiplexed in a digital time division multiplexed format. The standard for North America is the 24 channel T1 format.
The first stage of this process is the sampling of the analog (VF) signal to obtain a pulse amplitude modulated signal. Twenty-four of these channels are time multiplexed onto a common bus. These PAM samples are then encoded one at a time by the PCM encoder in the transmit unit of the channel bank.
The VF signal is band limited to 3400 Hz by a low pass active filter. Additionally 60 Hz filtering is needed in the channel bank. Currently a passive inductor/capacitor (LC) filter is provided for the 60 Hz filtering and the PAM samples are obtained in a bipolar gate. However, the gate and passive filtering prove to be expensive.
The present invention uses inexpensive integrated devices to reduce the cost. A novel circuit arrangement is provided to obtain the performance of the bipolar gate at reduced cost. 60 Hz filtering with this arrangement was accomplished by addition of a few additional passive components eliminating a 60 Hz filter stage prior to the bipolar gate and reducing cost by a large factor.
It was found that by incorporating an a-c coupling capacitor in the feedback loop of a PAM sampling gate, low output offset voltage (or pedestal) was obtained. In the prior art the a-c coupling capacitor was utilized to couple the output of the sampling gate to the following circuit. Further, the addition of another resistor-capacitor (RC) network at the input of the sampling gate enables the gate to coact as a three pole high pass filter.
Thus in accordance with the invention there is provided a pulse amplitude modulation sampling gate having an input and an output, a differential input operational amplifier, a first sampling switch connected between the output of said differential input operational amplifier and said output characterized in that said output, through a second sampling switch, is a-c coupled to the non-inverting input of said operational amplifier.
Further, in accordance with the invention there is provided a pulse amplitude modulation gate which coacts as a three pole high pass filter additionally including a resistor-capacitor network connected at the input .